Nozzle area measurement

ABSTRACT

The nozzle exit area of a variable nozzle jet engine is determined electrically rather than mechanically by mounting an antenna within the interior of a jet engine in the diffuser region or in the afterburner region adjacent the diffuser region and connecting the antenna to a microwave power oscillator to excite the interior of the engine to substantial resonance preferably at a frequency corresponding to about 3/4 wavelengths. The antenna characteristics are monitored and the change in the characteristics are related to a change in the nozzle exit area. The change in characteristic is used to drive a display to show the opening area. The signal representing nozzle area may be used with a signal representing static pressure at the nozzle entrance and a signal representing ambient static pressure, to compute gross thrust.

States Patent 11 1 Henshaw et al.

[ NOZZLE AREA MEASUREMENT [75] Inventors: Charles H. Henshaw, Ottawa,

Ontario; Benoit .lean, Boucherville, Quebec, both of Canada [73] Assignee: Control Data Canada, Ltd.,

Willowdale, Ontario, Canada [22] Filed: June ll, 1973 [21] Appl. No: 368356 Primary ExaminerM. Henson Wood, Jr. Assistant ExaminerMichael Y. Mar Attorney, Agent, or Firm-James A. Lamb 5 7 ABSTRACT The nozzle exit area of a variable nozzle jet engine is determined electrically rather than mechanically by mounting an antenna within the interior of a jet engine in the diffuser region or in the afterburner region adjacent the diffuser region and connecting the antenna to a microwave power oscillator to excite the interior of the engine to substantial resonance preferably at a frequency corresponding to about 3/4 wavelengths. The antenna characteristics are monitored and the change 33/125 W in the characteristics are related to a change in the nozzle exit area. The change in characteristic is used References Clted to drive a display to show the opening area. The signal UNITED STATES PATENTS representing nozzle area may be used with a signal 2,595,241 5 1952 Goble 33/125 w X representing Static Prfissure at the nozzle entrance and 3,121,955 2/1964 King 33/125 W X a signal representing ambient static pressure, to com- 3,433,021 3/1969 Kast 60/242 pute gross thrust. 3,604,210 9/1971 Johnson 60/242 X 17 Claims, 5 Drawing [Figures 5 c- M M Ti i t Y T f 7 Z14 if J5 RF MEASURiNG 21 Q0 EQUlPMENT NOZZLE AREA MEASUREMENT This invention relates to the determination of the area of the nozzle opening in a variable nozzle type jet engine.

It is desirable to know at all times the nozzle opening, that is the nozzle exit area, in a jet engine with a variable nozzle. The nozzle exit area is useful to the pilot, aircrew and engine maintenance crew of an aircraft, for example in setting and monitoring the engine and checking its operation. In addition, if the actual area could be determined by an inflight measurement, the aircrew could determine malfunctions of the mechanism which adjusts the nozzle and damage to the nozzle such as a bent or missing portion. Such an actual determination could also be used to ascertain if the thrust reversers were properly deployed during landing.

One of the most important uses for an accurate determination of nozzle area is in the calculation or determination of gross thrust and the invention will be described with this in mind. Very generally there are three variables involved in the determination of gross thrust. These variables are: the ambient static pressure P the nozzle total pressure P and the nozzle exit area A The ambient pressure P is conveniently available fronrthe flightinstumerrtation in the aircraft. However the nozzle total pressure P is difficult to determine t ut. 9. 195. an. itt tasrs ...nts zsta the nozzle exit area A8 is difficult to determine accurately in an operating engine. Various techniques have been devised in the past to determine gross thrust without a direct measurement of nozzle total pressure P or a direct 'measurementpf nozzle exit area A,,. One such method of determining nozzle total pressure without the use of an immersed probe, is disclosed for example in copending US. Pat. application Ser. No. 222,225 filed Jan. 31, 1972.

A very convenient technique or system for determining gross thrust is available if nozzle exit area can be determined with sufficient accuracy. This system is known and is referred to as the Engine Pressure Area Ratio system or EPAR, but requires a relatively exact determination of nozzle exit area. The need for accuracy is indicated, for example in Integrated Engine instrument System, Office of Naval Research, Dept. of

Navy, May 1969, 69-5081, where it says on page 6 2, Use of EPAR as a thrust indicator involves an accuracy deterioration caused by the difficulty of obtaining exact, effective exhaust nozzle area information. Net thrust errors in the order of up to 20 percent have been reported for supersonic cruise conditions.

At the present time, military jet engines and some civilian jet engines employ a variable nozzle which is adjusted or controlled to give optimum operation at given settings of throttle and afterburner fuel flow under different flight conditions. In most of these engines the variable nozzle comprises a plurality of vanes fastened to the tail pipe at the downstream end. A vane actuator, operated mechanically, electrically or hydraulically, positions the vanes to alter or adjust the exit area. The vanes may be hinged or may slide or run on track. It is obvious that information regarding nozzle area can be derived from the vane actuator position and this has been done in the past. However, the mechanical linkages are subject to thermally induced errors, wear and consequent lost motion, backlash, and variable friction factors. The net result of these uncontrolled factors resulted in a nozzle area exit signal which was not sufficiently accurate for use in the determination of gross thrust.

A more refined approach was proposed in the past, in which the nozzle vanes were each fitted with a small strut and pulley, over which a flexible cable was strung so as to encircle the vane assembly. The cable was fixed to one vane or leaf and was guided circumferentially around the vane assembly to the vane or leaf adjacent the one to which the cable was fixed. The cable was then led back to a spring tensioning device and a vane position transducer. In such a prior art arrangement the cable is always under tension which eliminates lost motion. However, the cable is subjected to considerable temperature change and this has an undesirable effect on accuracy. Moreover, the presence of the cable and struts disturbs the airflow over the outside of the vane assembly. This is of particular significance in a two stream engine where a disturbance of the outer flow wouldseriously degrade engine performance.

Neither of these prior art mechanical systems for indirect measurement can indicate changes in nozzle exit area by deformation or by expansion and contraction as a result of temperature effects, or severe changes in nozzle configuration such as might be caused by damage or loss of a nozzle vane. 7

It has also been suggested that certain optical arrangements might be used to determine nozzle exit area. These optical arrangements generally introduce severe problems of system complexity and signal-to noise ratio which tend to make them impractical for use in an aircraft jet engine, particularly for in-flight u e- The present invention provides means for a determination of nozzle exit area which does not involve mechanical measurement and avoids the errors inherent in m ben s lme ssr m nts s ms.

It is therefore an object of this invention to determine the nozzle exit area of a jet engine directly using electriselme sure t- It is another object of the invention to provide appa ratus for determining nozzle exit area which treats the engine tailpipe as a short waveguide.

It is another object of the invention to provide novel apparatus for determining the nozzle exit area of a jet engine which will function on the ground or in-flight.

It is yet another object of the invention to provide apparatus capable of indicating certain malfunctions of t h e variable nozzle mechanism.

It is yet another object of the invention to provide novel apparatus for determining jet engine gross thrust using as one input parameter a signal, derived by nonmechanical means, representing nozzle exitarea.

The present invention considers the after pan of a jet engine, from the turbine disc to the nozzle exit plane, as an open ended short wave guide in which a radio frequency standing wave field may be excited. The configuration of the field and its intensity will depend on frequency, power input level, and resistive and dielectric losses in the wave guide interior and walls, as well as power loss by radiation from the open end. Resonant cavities may have a great variety of configurations, but the one that resembles the tail pipe volume of a jet engine fairly closely is a circular wave guide stub closed at one end and having a variable aperture at the other end. A field configuration that would be suitable for such a geometry is known in waveguide theory as the TE mode. This field mode shows an ever decreasing attenuation factor with increasing excitation frequency above cut-off. This characteristic offers the advantage that thermally-induced wall resistivity changes affect the wave guide admittance components less severely than in other field modes. However, other modes, such as TM, and higher TE and TM modes, may be used.

The nozzle exit area represents an aperture in the downstream end of the engine interior or waveguide (as it may be referred to hereinafter) and will, depending on size, allow energy to escape from the waveguide. The amount of energy that escapes will depend on frequency and aperture size, and the changes or variations in the amount of energy lost will be reflected as variations in the characteristics of the shortwave guide, such as, for example, the storage factor Q. If the waveguide is excited by a coupled antenna or loop, the characteristics of the antenna (for simplicity a loop coupling may be included when reference is made to antenna) are in turn modified in terms of relative conductance and susceptance. The changes in antenna characteristic can be monitored and measured and are representative of changes in aperture area.

It was suggested in the past that ions and/or free electrons present in the exhaust gases of a jet engine would interfere with radio frequency measurements. However, experimental work done in connection with this invention tested radio frequency propagation in the exhaust gases at 400, 800, 1,000, 3,000, 5,000 and 10,000 megahertz and showed no appreciable attenuation caused by ionization in the hot gases.

Therefore the present invention is for a method for determining the area of a nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle, by exciting the interior region of the jet engine from the diffuser aft with an antenna within the region with a microwave signal to establish a substantially resonant condition, changing the size of the nozzle opening, and monitoring at least one characteristic of the antenna to derive a signal proportional to the change in characteristic and representative of the change in nozzle opening.

The invention is also for apparatus for determining the nozzle exit area in a jet engine having a diffuser, a tailpipe and a variable nozzle and including a source of microwave energy, an antenna connected to the source of microwave energy and mounted in the interior of the engine for exciting the interior of the engine with the microwave energy to establish a substantially resonant condition in the interior, and monitoring means connected to the antenna to monitor at least one characteristic thereof and having an output providing a signal related to the characteristic and representing nozzle exit area.

The invention will be described in more detail with reference to the accompanying drawings, in which FIG. I is a schematic sectional view of the aft part of a jet engine with a variable nozzle, showing apparatus according to the invention.

FIG. la is a schematic sectional view of a portion of the jet engine of FIG. 1 showing an alternative antenna structure.

FIG. 2 is a schematic block diagram of the measuring equipment.

FIG. 3 is a schematic sectional view of the aft part of a two stream type of jet engine with a variable nozzle and indicating the antenna means of the invention.

FIG. 4 is a schematic block diagram showing the invention as used to provide a determination of gross thrust.

Referring to FIG. 1 there is shown in schematic form the aft or rear portion of a jet engine of the afterburning type and having a variable exhaust nozzle. The engine has a diffuser region A, a tailpipe or afterburning region B, and a nozzle region C. A rear bullet or afterbody 11 is in the diffuser region A to assist in establishing a favourable air flow through the engine. It is supported by struts 12. In the tailpipe region B there is a tailpipe l4 and a flameholder or gutters 15 for the afterburner. The flameholder 15 is in the general form of concentric V-rings or gutters and two are shown. In the nozzle region C there is a variable nozzle 16 which has an open position shown by solid lines and a closed or choked position shown by broken lines. The variable nozzle 16 is normally formed of leaves or vanes which may be hinged or slidably connected to vary the nozzle exit area indicated as A The operating mechanism for the vanes of a variable nozzle is not shown as such mechanisms are well known.

The engine described so far is typical of engines known in the prior art.

An antenna 17 is installed coaxially at the end of bullet 11 with a coaxial conductor 18 connecting the antenna with the radio frequency measuring equipment 20 which provides an output signal driving an indicator 21 showing the nozzle area A The antenna 17 is suitable when using the TM mode as will be referred to hereinafter.

FIG. la is an alternative embodiment and shows a part of the diffuser and afterburner. An antenna 17a (which is shown as a coupling loop) is in the form of a portion of a ring and is mounted in the afterbumer region spaced inwardly from casing 14. The coupling loop type of antenna 17a may extend around only a portion of theperiphery and have its remote or terminal end grounded and its other end connected to line 18. The antenna 17a is preferably mounted adjacent or just downstream of the afterburner. This mounting location places the antenna just outside of the hostile afterbumer environment which fans out from the flameholders 15. It is, of course, very desirable to position the antenna where it is not immersed in the afterbumer gases. This ring type antenna 17a is suitable for operating in the TE mode which is the preferred mode as it offers advantages in the form of reduced losses and attenuation of the field. In addition, when using this antenna, the frequency may be selected and the flameholders designed accordingly so that the flameholders are caused to resonate. This tends to isolate the engine cavity forward of the flameholders.

Referring now to FIG. 2, there is shown in block form, circuitry according to one embodiment as an example of suitable circuitry for RF measuring equipment of block 20. The circuitry shown employs a frequency modulated signal and includes a sweep generator 22 connected to a voltage controlled microwave power oscillator 23 so that the generator 22 causes the output of power oscillator to sweep through a desired frequency range from one side to the other. The desired frequency or centre frequency is that frequency which gives substantial resonance within the tailpipe at a low integral odd number of quarter wavelengths, for example, three quarter wavelength. It should be noted that in some engine configurations it may be desirable to use a centre frequency which departs somewhat from the low integral odd number of quarter wavelengths, for example to avoid interaction at the exit plane in a two stream engine. The modulated energy from the power oscillator 23 is applied to an isolator 24 and thence to two transmission lines 18 and 25. The isolator 24 transmits the electrical energy preferentially in one direction so that reflected energy is not appreciably coupled back into the power oscillator 23. The two transmission lines 18 and 25 may be referred to as the active line and reference line, respectively. The two transmission lines are of the same type and are made the same length. The reference line 25 is short circuited and the active line 18 goes to the antenna. The intrinsic transmission line characteristics of the two lines will be closely similar and the signals from the reference line 25 can be used to balance out the intrinsic portion of the signal on the active line 18. This provides a residual signal which is functionally related to the tailpipe characteristics. Thus, the transmission lines 18 and 25 are both connected to a block 26 representing signal processing and measuring circuitry.

A local oscillator 27 is phase-locked to the power oscillator 23 so that it accurately tracks the output of the power oscillator 23 over the modulation cycle. The output of local oscillator 27 is fed to the signal processing circuitry for IF amplifiers. Thus, the residual signal (the signal after balancing out or cancelling of substantially all the intrinsic portion of the signal from transmission line 18) is amplified and passed to a detector which detects changes in at least one of the admittance components of the signal. It has been found experimentally that both conductance and susceptance change with a change in the nozzle area and that under certain conditions susceptance changes quite significantly and may be calibrated to show nozzle area. Under some circumstances both conductance and susceptance could be used. The signal representing change in conductance or susceptance is the output of block and may be used to drive indicator 21 to show nozzle area.

As an example, it was found that using a frequency of 830 megahertz and an initial nozzle opening of 130 square inches gave initial readings balance readings of 9.5 millimhos conductance and 16.6 millimhos susceptance. A change in nozzle area of /2 percent (approximately 0.65 square inches) gave readings of 9.4 millimhos conductance and 16.9 millimhos susceptance. That is, a change in area of V2 percent gave a change in susceptance of 0.3 millimhos, a sufficiently large change to be readily usable for an indicator.

Slightly different circuitry would be used in an embodiment employing two switched frequencies from the power oscillator instead of a continuously sweeping output. This would provide square wave amplitude and phase signals in the processing circuitry. While this embodiment would use somewhat simpler circuitry, the frequency stability becomes more critical.

Yet another embodiment would use a single fixed frequency as an output from the power oscillator, and this would produce a DC signal in the processing circuitry. The DC signal would be a function of phase. The equipment would be simpler but susceptible to drift and perhaps more sensitive to noise.

Referring to FIG. 3 there is shown the rear or aft part of a two stream type ofjet engine where the inner portion is substantially the same as the rear portion of the engine of FIG. 1. There is an outer casing 30 which provides a passageway 31 for the flow of cool air which is guided past variable nozzle 16 and exits through the secondary nozzle 32. An antenna 17 is shown mounted on bullet 11 as before and functions in the same way as described in connection with FIGS. 1 and 2. It will of course be apparent that antenna 17a of FIG. 1a could also be used. The area A is the area measured while the area A, represents the opening in the secondary nozzle 32.

The apparatus described will determine the nozzle exit area A with accuracy not heretofore achieved in jet engines under operating conditions.

The value Ag may be used in the determination of gross thrust, and this use of the invention will be de scribed.

For a simple convergent nozzle the gross thrust of the jet engine may be defined as:

F A 'P 'g, [(P -/P for incomplete expansion FG s so'gt' l( T.\/ SO)] for Complete expansion where:

F gross thrust A nozzle exit area P ambient static pressure P nozzle total pressure Pry/P nozzle pressure ratio g, [(P /P H a function of nozzle pressure ratio for incomplete expansion g [(P -/P a function of nozzle pressure ratio for complete expansion For a convergent-divergent type nozzle there is a similar relationship and only complete expansion need be considered as this type of nozzle can ideally expand the exhaust gases to ambient static pressure. The gross thrust may be defined as:

r; a' sufc HPTNIJPSUH where:

F gross thrust A nozzle exit area at station 9 which is the final opening plane or station (the opening of the divergent portion of a convergent-divergent nozzle). P ambient static pressure P nozzle total pressure fi- [(P /P H a function of the nozzle pressure ratio 1f the expansion is complete at station 9 it will not be complete at station 8 which is the smallest part of the nozzle (i.e., the area defined by the convergent portion), then we can rewrite equation (3) as follows:

where:

F primary gross thrust A nozzle exit area at station 8 P ambient static pressure f,[ (P /P 3] a function of the nozzle pressure ratio The two stream engine of FIG. 3 requires a slightly different treatment. Since the primary purpose of the outer stream is normally for cooling, the gross thrust determination in flight is concerned with the thrust produced by nozzle 16. The primary exhaust gases are not fully expanded at station 8, which is the engine exit station. Use is therefore made of the following equation to determine the primary gross thrust:

where:

P primary gross thrust A nozzle exit area P ambient static pressure f, [(Pm/P H a function of nozzle pressure ratio at station 8 for a choked convergent nozzle.

Thus it will be seen that an accurate determination of the nozzle exit area at station 8 may be used to determine gross thrust for any type of nozzle configuration. The nozzle total pressure can be measured with a probe in a non-afterburning engine, and in an afterburning engine may be determined as will be described hereinafter.

It is useful to know nozzle total pressure when the afterburner is operating as this may be used in the relationship previously indicated, along with nozzle area, to determine gross thrust. Because an immersed total pressure probe has a very short life when subjected to an afterburning environment, it is very desirable to determine nozzle total pressure in another manner. Prior teachings have revealed a simple relationship linking nozzle total pressure P to the jet engine primary exhaust area A and nozzle entrance static pressure P This relationship is Therefore, the afterburning primary gross thrust can be calculated using the following relationship:

or 30%, s0 sx) Referring now to FIG. 4, there is shown, in block form, apparatus for determining gross thrust ofa jet engine. A signal representing nozzle exit area A is available on conductor 40. A tap 41 (FIG. 1) is connected to pressure transducer 42 which provides a signal representing static pressure at the nozzle entrance P on conductor 43. A pressure transducer 44 is connected with the aircraft instrumentation to provide a signal representing ambient static pressure P on conductor 45. These are the input signals. Block 46 represents computer circuitry which receives signals representing P and A and it solves equation (6) above to provide on conductor 47 a signal representing P Block 48 represents computer circuitry which receives signals on conductors 40 and 47 representing A and P and on conductor 45 representing P to solve equation (7) above and provide an output signal representing gross thrust which may be used to drive a display indicator 50.

The value of A must be determined with considerable accuracy to be useful in determining gross thrust in this manner, and the prior art mechanical measurement of A,, was not able to achieve sufficient accuracy.

It will be apparent to those skilled in the art that other excitation modes and arrangements could be used to excite the interior regions of a jet engine according to the invention.

We claim:

1. A method for determining the area of the nozzle opening in ajet engine having a diffuser, a tailpipe and a variable nozzle, comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal to establish a substantially resonant condition,

changing the size of the nozzle opening, and

monitoring at least one characteristic of the antenna to derive a signal proportional to the change in characteristic and representative of the change in nozzle opening.

2. A method as defined in claim 1 in which the antenna is mounted on the diffuser bullet and the interior region of the engine is excited in the TM mode.

3. A method as defined in claim 1 in which the antenna is in the form of a portion of a ring adjacent the casing and in the region of the flameholder but upstream of the flameholder gases, nd the interior region is excited in the TE mode.

4. A method as defined in claim 1 in which the antenna characteristic is selected from the group comprising conductance and susceptance.

5. A method for determining the area of a nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal establishing substantial resonance within the region at a low integral odd number of quarter wavelengths, changing the size of the nozzle opening,

monitoring at least one characteristic of the antenna to derive a signal proportional to the change in the characteristic and representative of the change in nozzle opening.

6. A method for determining the area of a nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal at a frequency establishing substantial resonance within said region at a low integral odd number of quarter wavelengths,

sweeping said microwave signal above and below said frequency to provide a frequency modulated signal,

changing the size of the nozzle opening,

monitoring the change in susceptance characteristic of the antenna, and

providing a signal representing change in susceptance and representative of nozzle opening.

7. Apparatus for determining nozzle exit area in a jet engine having a diffuser, a tailpipe and a variable nozzle, said apparatus comprising a source of electromagnetic microwave energy, antenna means connected to said source of microwave energy and mounted in the interior of said engine for exciting the interior of the engine, including at least the interior defined by said tailpipe, with said microwave energy to establish a substantially resonant condition in said interior,

monitoring means connected to said antenna means for monitoring at least one characteristic thereof and having an output providing a signal related to said characteristic and representing the nozzle exit area.

8. Apparatus for determining nozzle exit area in ajet engine having a diffuser, a tailpipe and a variable nozzle, said tailpipe defining at least part of an interior region of said engine, said variable nozzle having a plural ity of movable vanes defining said nozzle exit area, said apparatus comprising a source of microwave energy,

antenna means connected to said source and mounted within said interior region for exciting said interior region with microwave energy at a predetermined frequency to establish a substantially resonant condition within said interior region at a low integral odd number of quarter wavelengths,

monitoring means connected to said antenna means for monitoring at least one characteristic of said antenna means and having an output providing a signal related to said characteristic and representing nozzle exit area.

9. Apparatus as defined in claim 8 in which said antenna means is an antenna mounted centrally within the walls of the diffuser and projecting into said interior region, for excitation of said region in the TM mode.

10. Apparatus as defined in claim 8 in which said antenna means is an antenna comprising a portion of a circularly formed ring, mounted within said interior region and having its periphery spaced from the walls defining said region, for excitation of said region in the TE mode.

11. Apparatus as defined in claim 10 and including a flameholder within said tailpipe having outer gutters of a predetermined diameter, said gutters being spaced substantially equidistant from said walls, the ring portion of the antenna having a diameter greater than said predetermined diameter and being mounted radially outwardly of said outer gutters and longitudinally between a position opposite said outer gutters and a position downstream of said gutters and upstream of the hot flameholder gas region.

12. Apparatus as defined in claim 8 in which said low integral odd number of quarter wavelengths is three quarter wavelengths.

13. Apparatus as defined in claim 8 in which said one characteristic of said antenna is selected from the group comprising conductance and susceptance.

14. Apparatus as defined in claim 13 in which said one characteristic is susceptance.

15. Apparatus for determining the nozzle exit area A in a jet engine having a diffuser, a tailpipe and a variable nozzle, said tailpipe defining at least part of an interior region of said engine, said variable nozzle having a plurality of movable vanes defining a nozzle exit area, said apparatus comprising a voltage controlled microwave power oscillator,

a sweep generator connected to said oscillator to cause the output of said microwave oscillator to sweep continuously across a predetermined center frequency,

an antenna mounted in said interior region for exciting said interior region with microwave energy, modulated by said sweep generator, at said predetermined center frequency to establish a substantially resonant condition within said interior region,

an active and a reference transmission line of substantially equal length, each connected at one end to the output of said oscillator, the other end of said reference transmission line being short circuited and the other end of said active line being connected to said antenna,

signal processing means coupled to said active and said reference transmission lines and including means for balancing signals on said transmission lines to cancel intrinsic signals present on both lines to provide a residual signal representing a characteristic of said antenna exciting said interior region, said signal processing means also including an emplifier to amplify said residual signal and a detector to detect changes in at least one of the admittance components of said signal, said changes being related to the nozzle exit area, and providing an output signal related to the detected signal and representing nozzle exit area A 16. Apparatus as defined in claim 15 and further ineluding apparatus for utilizing said output signal representing nozzle exit area A for determining gross thrust of said engine and providing a final output signal representing gross thrust comprising first pressure responsive means for detecting static pressure at the nozzle entrance P and for providing a signal representing P first computer means connected to said signal processing means and to said first pressure responsive means for receiving signals representing A and P and for providing a signal representing total pres sure at the nozzle entrance P according to the relationship second pressure responsive means for detecting ambient static pressure P and for providing a signal representing P second computer means connected to said signal processing means, to said first computer means, and to said second pressure responsive means for receiv ing signals representing A and P and P and for providing a final output signal representing gross thrust F according to the relationship r; 8( u, ry s0)- ll7. Apparatus for determining nozzle exit area in a jet engine having a diffuser, a tailpipe and an exit nozzle, said apparatus comprising a source of electromagnetic microwave energy,

antenna means connected to said source of microwave energy and mounted in the interior of said engine for exciting the interior of "the engine, including at least the interior defined by said tailpipe,

with said microwave energy to establish a substanand having an output providing a signal related to tially resonant condition in'said interior, said characteristic and representing the nozzle exit monitoring means connected to said antenna means area.

for monitoring at least one characteristic thereof 

1. A method for determining the area of the nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle, comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal to establish a substantially resonant condition, changing the size of the nozzle opening, and monitoring at least one characteristic of the antenna to derive a signal proportional to the change in characteristic and representative of the change in nozzle opening.
 2. A method as defined in claim 1 in which the antenna is mounted on the diffuser bullet and the interior region of the engine is excited in the TM01 mode.
 3. A method as defined in claim 1 in which the antenna is in the form of a portion of a ring adjacent the casing and in the region of the flameholder but upstream of the flameholder gases, nd the interior region is excited in the TE01 mode.
 4. A method as defined in claim 1 in which the antenna characteristic is selected from the group comprising conductance and susceptance.
 5. A method for determining the area of a nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal establishing substantial resonance within the region at a low integral odd number of quarter wavelengths, changing the size of the nozzle opening, monitoring at least one characteristic of the antenna to derive a signal proportional to the change in the characteristic and representative of the change in nozzle opening.
 6. A method for determining the area of a nozzle opening in a jet engine having a diffuser, a tailpipe and a variable nozzle comprising the steps of exciting the interior region of a jet engine from the diffuser aft by excitation from an antenna within said region with a microwave signal at a frequency establishing substantial resonance within said region at a low integral odd number of quarter wavelengths, sweeping said microwave signal above and below said frequency to provide a frequency modulated signal, changing the size of the nozzle opening, monitoring the change in susceptance characteristic Of the antenna, and providing a signal representing change in susceptance and representative of nozzle opening.
 7. Apparatus for determining nozzle exit area in a jet engine having a diffuser, a tailpipe and a variable nozzle, said apparatus comprising a source of electromagnetic microwave energy, antenna means connected to said source of microwave energy and mounted in the interior of said engine for exciting the interior of the engine, including at least the interior defined by said tailpipe, with said microwave energy to establish a substantially resonant condition in said interior, monitoring means connected to said antenna means for monitoring at least one characteristic thereof and having an output providing a signal related to said characteristic and representing the nozzle exit area.
 8. Apparatus for determining nozzle exit area in a jet engine having a diffuser, a tailpipe and a variable nozzle, said tailpipe defining at least part of an interior region of said engine, said variable nozzle having a plurality of movable vanes defining said nozzle exit area, said apparatus comprising a source of microwave energy, antenna means connected to said source and mounted within said interior region for exciting said interior region with microwave energy at a predetermined frequency to establish a substantially resonant condition within said interior region at a low integral odd number of quarter wavelengths, monitoring means connected to said antenna means for monitoring at least one characteristic of said antenna means and having an output providing a signal related to said characteristic and representing nozzle exit area.
 9. Apparatus as defined in claim 8 in which said antenna means is an antenna mounted centrally within the walls of the diffuser and projecting into said interior region, for excitation of said region in the TM01 mode.
 10. Apparatus as defined in claim 8 in which said antenna means is an antenna comprising a portion of a circularly formed ring, mounted within said interior region and having its periphery spaced from the walls defining said region, for excitation of said region in the TE01 mode.
 11. Apparatus as defined in claim 10 and including a flameholder within said tailpipe having outer gutters of a predetermined diameter, said gutters being spaced substantially equidistant from said walls, the ring portion of the antenna having a diameter greater than said predetermined diameter and being mounted radially outwardly of said outer gutters and longitudinally between a position opposite said outer gutters and a position downstream of said gutters and upstream of the hot flameholder gas region.
 12. Apparatus as defined in claim 8 in which said low integral odd number of quarter wavelengths is three quarter wavelengths.
 13. Apparatus as defined in claim 8 in which said one characteristic of said antenna is selected from the group comprising conductance and susceptance.
 14. Apparatus as defined in claim 13 in which said one characteristic is susceptance.
 15. Apparatus for determining the nozzle exit area A8 in a jet engine having a diffuser, a tailpipe and a variable nozzle, said tailpipe defining at least part of an interior region of said engine, said variable nozzle having a plurality of movable vanes defining a nozzle exit area, said apparatus comprising a voltage controlled microwave power oscillator, a sweep generator connected to said oscillator to cause the output of said microwave oscillator to sweep continuously across a predetermined center frequency, an antenna mounted in said interior region for exciting said interior region with microwave energy, modulated by said sweep generator, at said predetermined center frequency to establish a substantially resonant condition within said interior region, an active and a reference transmission line of substantially equal length, each connected at one end to the output of said oscillator, the other End of said reference transmission line being short circuited and the other end of said active line being connected to said antenna, signal processing means coupled to said active and said reference transmission lines and including means for balancing signals on said transmission lines to cancel intrinsic signals present on both lines to provide a residual signal representing a characteristic of said antenna exciting said interior region, said signal processing means also including an emplifier to amplify said residual signal and a detector to detect changes in at least one of the admittance components of said signal, said changes being related to the nozzle exit area, and providing an output signal related to the detected signal and representing nozzle exit area A8.
 16. Apparatus as defined in claim 15 and further including apparatus for utilizing said output signal representing nozzle exit area A8 for determining gross thrust of said engine and providing a final output signal representing gross thrust comprising first pressure responsive means for detecting static pressure at the nozzle entrance PSN and for providing a signal representing PSN, first computer means connected to said signal processing means and to said first pressure responsive means for receiving signals representing A8 and PSN and for providing a signal representing total pressure at the nozzle entrance PTN according to the relationship PTN PSN.f(A8) second pressure responsive means for detecting ambient static pressure PSO and for providing a signal representing PSO, second computer means connected to said signal processing means, to said first computer means, and to said second pressure responsive means for receiving signals representing A8 and PTN and PSO and for providing a final output signal representing gross thrust FG according to the relationship FG g(A8, PTN, PSO).
 17. Apparatus for determining nozzle exit area in a jet engine having a diffuser, a tailpipe and an exit nozzle, said apparatus comprising a source of electromagnetic microwave energy, antenna means connected to said source of microwave energy and mounted in the interior of said engine for exciting the interior of the engine, including at least the interior defined by said tailpipe, with said microwave energy to establish a substantially resonant condition in said interior, monitoring means connected to said antenna means for monitoring at least one characteristic thereof and having an output providing a signal related to said characteristic and representing the nozzle exit area. 